1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and a method for forming the image. In particular, the present invention relates to the image forming apparatus using a simultaneous transfer and fixing system in which a toner image formed on an image carrier is heated and pressed to be fixed on a sheet simultaneously with a transfer of the image via an intermediate transfer member.
2. Description of the Related Art
A conventional image forming apparatus uses an electrophotographic system to form a favorable color image such as described below.
Such an image forming apparatus includes the same number of image carriers (i.e., photosensitive members) as kinds of color required in image formation, a charging unit disposed around the image carriers, an exposure unit, and a developing unit. The image forming apparatus superimposes and transfers (a primary transfer) single-color toner images formed on each image carriers onto the intermediate transfer member or a sheet to form a color image.
In general, an image forming apparatus using an intermediate transfer member electrostatically transfers the toner image from the intermediate transfer member onto a sheet. However, sometimes a problem arises when a multi-color image formed on the intermediate transfer member is transferred onto the sheet, i.e., when performing a secondary transfer process.
In the secondary transfer, a toner image on an intermediate transfer member is transferred onto various types of sheets. If a toner image is to be transferred onto a sheet whose surface is greatly uneven, a gap between the intermediate transfer member and the sheet at the transfer position becomes also uneven, so that a transfer electric field is distorted. As a result, the toner is dispersed and the transfer is not correctly performed.
Further, since an amount of moisture in a sheet greatly affects transferability, image formation may not be stably performed due to environmental changes such as a change in humidity.
Further, toner images of a plurality of colors are superimposed and formed on an intermediate transfer member. Therefore, for example, while a toner image of three or more layers is formed on one position, a toner image of one layer may be formed on another position. Consequently, a thickness of the toner image varies according to a position, or a charge amount for each color image becomes uneven. As a result, it is difficult to uniformly apply an electric field on the toner image, so that an abnormal image can be generated on an intermediate transfer member where a toner image is thick or thin, or where a toner charge amount is large or small.
To address the above-described problem, Japanese Patent Application Laid-Open No. 10-63121 discusses an image forming apparatus using a simultaneous transfer and fixing system. Such an image forming apparatus transfers a toner image formed on an image carrier onto an intermediate transfer member and heat-fuses the toner image formed on the intermediate transfer member. The heat-fused toner image is then pressed onto a sheet to be fixed simultaneously with the transfer.
The heat-fused toner image of the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 10-63121 shows a more favorable transferability as compared to an electrostatic transfer system. The transferability is more favorable owing to a difference of surface energies between the intermediate transfer member and the sheet, a difference of effective contact areas of transferred toner on both sides, and adhesive force of the fused toner.
Moreover, Japanese Patent Application Laid-Open No. 2005-31312 discusses an image forming apparatus which transfers a toner image formed on an image carrier onto a first intermediate transfer member (i.e., a primary transfer), and transfers the toner image on the first intermediate transfer member onto a second intermediate transfer member (i.e., a secondary transfer). The image forming apparatus then heats and presses the toner image formed on the second intermediate transfer member to transfer and fix the image on a sheet.
The image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2005-31312 includes a transfer member contacting/separating unit that press-contacts and separates the first intermediate transfer member and the second intermediate transfer member. Consequently, a temperature rise in the image carrier caused by the intermediate transfer members is controlled, and image degradation due to a temperature rise is reduced.
On the other hand, a fixing apparatus generally uses as a heat source a heating member disposed in a longitudinal direction (i.e., direction of a roller shaft) inside a heating roller. A surface temperature of the heating roller is controlled to be at a desired temperature by measuring a surface temperature of the heating roller and controlling an ON/OFF state of the heating member according to the measurement result.
However, the surface temperature of the heating roller changes due to various causes, so that it is difficult to accurately maintain a constant surface temperature.
For example, when a sheet is passed through a fixing apparatus, the sheet takes off heat and the temperature on a surface of a heating roller becomes uneven. In particular, if short sheets are continuously passed through the fixing apparatus in the longitudinal direction of the heating roller, heat is taken off only from a portion where the sheets pass, thereby generating a difference in temperature distribution in the longitudinal direction of the heating roller. As a result, an edge temperature rises in the heating roller, i.e., a temperature greatly rises at a portion of the heating roller where the sheets do not pass. The edge temperature rise may lead to image degradation such as high temperature offset or uneven brightness.
To solve such a problem, Japanese Patent Application Laid-Open No. 06-332338 discusses a technique by which a heating member inside a heating roller is segmented in a longitudinal direction. Power distribution of the segmented heating member is switched and controlled respectively, so that the edge temperature rise and temperature unevenness can be reduced.
A temperature unevenness can also be generated in a transfer-fixing portion of the image forming apparatuses discussed in Japanese Patent Application Laid-Open No. 10-63121 and No. 2005-31312. In such a case, even if technique discussed in Japanese Patent Application Laid-Open No. 06-332338 reduces the temperature unevenness to a level which does not lead to image degradation, there arises a problem as described below.
As long as there is a temperature difference in the transfer-fixing portion of the image forming apparatus, an intermediate transfer member expands and contracts due to a difference in a heat expansion rate. As a result, magnification of a toner image formed on the intermediate transfer member changes according to expansion/contraction of the intermediate transfer member. Such a toner image is then directly transferred and fixed onto a sheet, so that a departure in image magnification occurs on the sheet.